Examples of prior art superconducting mixers are described in publications such as J.S. Martens, "The Models and Applications of a High Frequency Three Terminal Device Made of High Temperature Superconductors," Ph.D. Thesis, University of Wisconsin, Madison, 1990, pgs. 254-259 and J.S. Martens et at, S-Parameter Measurements and Applications of Superconducting Flux Flow Transistors," IEEE MTT-S Digest, pgs. 1231-1234. The prior art mixers generally may be described as a mixer which includes two flux flow transistors in a cascode configuration which has a feed back between input and outputs ports. This prior art mixer, however, is relatively large in size, is relatively unstable and is likely to oscillate because of its feed back path.
Other related prior art publications include R.S. Pengelly, "Microwave Field Effect Transistors--Theory, Design and Applications," Research Studies Press, 1982, Pgs. 233-236 and J.H. Hinken, Superconductor Electronics, Springer-Veriag, 1989, pgs. 4-12.